The present invention generally relates to an image forming apparatus, and more particularly to an image forming apparatus such as a laser beam printer capable of being applied to digital copy machines and the like, and preferably applied to digital color copy machines.
Laser beam printers are manufactured based on an electrophotography technology and a laser beam scanning technology. As a high quality image can be rapidly obtained, use of the laser beam printers as output devices of computers and printer units of digital copy machines has spread. In the laser beam printer, to obtain a high quality image, a density for every dot is controlled. When the density for each dot is controlled, an image having good gradation and resolution can be obtained. There are two types of methods for controlling the density for each dot. A first method modulates an intensity of a laser beam emitted from a semiconductor laser unit in accordance with image information. This first method is often referred to as beam intensity modulation. A second method modulates a width of a driving pulse signal, which drives the semiconductor laser unit, in accordance with the image information. This second method is often referred to as pulse width modulation.
In pulse width modulation, as the semiconductor laser unit is controlled so as to be only turned on or off, it is possible to stably record an image. However, when a scanning speed of the laser beam increases (a frequency of a writing clock increases), a minimum pulse width decreases. For example, in a case where gradational image data having 256 levels is represented for each dot, when the frequency of the writing clock is 20 MHz, the minimum pulse width is approximately 0.2 nsec. In this case, it is difficult to control the pulse width at the minimum pulse width.
On the other hand, in beam intensity modulation, for example, due to a light-electricity negative feed back loop in which an intensity variation of the laser beam is rapidly fed back to the driving signal of the semiconductor laser beam, the intensity of the laser beam can be correctly controlled in accordance with the image information. In this case, the gradational image having 256 levels can be represented for each dot at a writing clock having a frequency of 25 MHz. However, in a case where an image is formed by use of an electrophotography process, as the laser beam emitted from the semiconductor laser unit is a Gaussian beam so that adjacent dots formed by the laser beam overlap each other, beam intensity modulation has the following disadvantages.
(1) Due to a speed variation of a photosensitive member (a recording medium) moving in a predetermined direction, the density of the image varies in the moving direction of the photosensitive member. As the laser beam is a Gaussian beam, when the speed of the photosensitive member varies, an overlapped area in which adjacent dots are overlap each other varies. As a result, the amount of toner adhered to the overlapped area on the photosensitive member varies, so that the density of the image varies in the moving direction of the photosensitive member.
(2) Due to a position variation of an optical device such as a polygonal mirror, the density of the image varies. When the position of the optical device varies, the position of the beam spot projected onto the surface of the photosensitive member varies. As a result, the amount of toner adhered to the overlapped area on the photosensitive member varies, so that the density of the image varies.
(3) In a case where a low density dot is formed, an electrical potential curve (a potential distribution) at an area, corresponding to the low density dot, on the surface of the photosensitive member has a gently sloping shape. Thus it is difficult to constantly adhere the toner to areas corresponding to low density dots, so that it is difficult to form the low density dots with a constant density. That is, the reproducibility of the density for the low density dots deteriorates.